New submission from Aaron Ecay <[email protected]>:
I have discovered that InitVar's are passed in a surprising way to the
__post_init__ method of python dataclasses. The following program illustrates
the problem:
=====
from dataclasses import InitVar, dataclass
@dataclass
class Foo:
bar: InitVar[str]
quux: InitVar[str]
def __post_init__(self, quux: str, bar: str) -> None:
print(f"bar is {bar}; quux is {quux}")
Foo(bar="a", quux="b")
=====
The output (on python 3.7.3 and 3.8.0a3) is (incorrectly):
bar is b; quux is a
This behavior seems like a bug to me, do you agree?
I have not looked into the reason why it behaves this way, but I suspect that
the InitVar args are passed positionally, rather than as key words, to
__post_init__. This requires the order of arguments in the definition of
__post_init__ to be identical to the order in which they are specified in the
class. I would expect the arguments to be passed as keywords instead, which
would remove the ordering dependency. If there is agreement that the current
behavior is undesirable, I can look into creating a patch to change it.
----------
components: Library (Lib)
messages: 356125
nosy: Aaron Ecay
priority: normal
severity: normal
status: open
title: Surprising and possibly incorrect passing of InitVar to __post_init__
method of data classes
versions: Python 3.7
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Python tracker <[email protected]>
<https://bugs.python.org/issue38719>
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